This application claims priority to European Patent Application No. 01200236.6 filed Jan. 23, 2001.
The present invention relates to a DC/DC converter comprising inductive electrical energy storage means, switching means and control means wherein said control means are arranged for selectively operating said switching means for transferring an amount of electrical energy from said energy storage means to an output of said DC/DC converter, for providing a desired output voltage.
A DC/DC converter of this type is known from International patent application WO 95/34121 in the name of applicant.
In practice, a DC/DC converter of the above type can be operated in a continuous or PWM (Pulse Width Modulation) mode, wherein electrical energy is continuously stored in the energy storage means, or in the discontinuous or PFM (Pulse Frequency Modulation) mode, wherein the energy storage means may be completely discharged.
In a single output DC/DC converter operated in PWM mode, a typical switching cycle comprises a first phase wherein the switching means are controlled for storing energy in the energy storage means, and a second phase wherein the switching means are controlled for transferring energy from the energy storage means to the output of the converter. The output power of a DC/DC converter operated in PWM mode is controlled by its duty cycle, which is the ratio of the length in time of the first phase and the total length in time of the switching cycle, i.e. the sum of the first and second phases.
For a digitally controlled DC/DC converter the length of the phases is defined by numbers which are derived from a counter running on a fixed clock frequency. Accordingly, the duty cycle is quantised, such that there is a limited number of possible duty cycles. Dependent on the clock frequency and the switching frequency of the converter, N different numbers of duty cycles can be provided, running from zero through 1/N, 2/N till N/N, i.e. 1. Accordingly, the duty cycles may only very in discrete steps. It will be appreciated that there is a control problem if, starting from an unknown input voltage, an exact defined output voltage has to be provided.
In the above-mentioned prior art document, this control problem is solved by allowing some variation in the output voltage, which may very within a voltage window. However, this solution has as a draw-back in that the accuracy of the output voltage is decreased, while the output voltage may continuously very.
As an alternative, the duty cycle of the converter can be stepwise varied, whether the output voltage is above or below a set reference voltage. However, this type of control is rather slow, resulting in a too large instability, such that the duty cycle and the output voltage may vary substantial very continuously. To solve this problem, a waiting time can be introduced between the adaptation of the duty cycle and a next measurement of the output voltage, however this too increases the slowness of the control.
It is an object of the present invention to provide a DC/DC converter of the above-mentioned time, which does not suffer from strong variations in the duty cycle, however anticipating very quickly to variations in the output voltage of the converter.
This object is solved in a DC/DC converter according to the present invention, characterized by digital control means which are configured for operatively controlling the switching means for transferring electrical energy in accordance with a switching sequence comprising a ramp-up switching cycle and a ramp-down switching cycle for substantially charging and decharging of the energy storage means.
In accordance with the present invention, the duty cycle of a switching cycle is adopted such that the amount of electrical energy stored in the energy storage means is substantially increased or decreased, dependent whether the output voltage is below or above its reference voltage, respectively.
As a consequence, the output voltage of the DC/DC converter will be substantially higher or lower than the reference voltage.
In a preferred embodiment of the invention, the control means are arranged for comparing the output voltage with a reference voltage, and wherein the switching means are controlled such that if the output voltage is below said reference voltage, a ramp-up switching cycle is used as a next switching cycle, and if the output voltage is higher than the reference voltage a ramp-down switching cycle is used as a next switching cycle.
After a ramp-down switching cycle, the output voltage is relatively low, as a result of which the control means will decide to start a ramp-up switching cycle. After this ramp-up switching cycle the output voltage will be too high, which leads to a ramp-down action in the next switching cycle, etc. During a ramp-up cycle the duty cycle of the converter is increased, whereas during a ramp-down cycle the duty cycle of the converter is decreased.
In a relatively easy to implement embodiment of the invention, the switching means are controlled in accordance with a duty cycle defined by the charging time of the energy storage means divided by the sum of the charging time and the decharging time of the energy storage means, wherein during a ramp-up switching cycle, the duty cycle is increased and wherein during a ramp-down cycle the duty is decreased.
In a yet further embodiment of the invention, the duty cycle is increased by increasing the charging time of the energy storage means and the duty cycle is decreased by increasing the discharging time of the energy storage means.
As an advantage, with the control of the converter according to the present invention, the resolution of the length in time of the first and second phases, i.e. the charging time and discharging time of the energy storage means, may be limited, because the accuracy of the output voltage is, among others, determined by the behaviour in time of the converter. The control according to the invention does not suffer from instabilities.
The invention may be practiced with a plurality of DC/DC converter designs, such as DC/DC up-converters, DC/DC down-converters, DC/DC up/down converters, DC/DC inverting converters, DC/DC converters with positive and negative output, wherein said DC/DC converters may comprise multiple outputs.
The invention also relates to a power supply comprising a DC/DC converter disclosed above, and arranged for receiving an input voltage at input terminals and for providing a controlled output voltage at output terminals of the converter. The DC/DC converter according to the invention is of a particular advantage if applied in an electronical appliance, such as, but not limited to, a portable electronic appliance.
In a preferred embodiment, the switching means comprise semiconductor switching means, in particular MOS (Metallic Oxide Semiconductor) transistor means.